(1) Field of the Invention
The present invention relates to a cellulose dope, a process for the preparation of this dope, a shaped article formed from this dope, and a method for the application of this dope.
(2) Description of the Related Art
Shaped articles of regenerated celluloses are ordinarily prepared by dissolving cellulose in a solvent by appropriate means and introducing the obtained dope into a nonsolvent or regenerating solvent by using an extruder. Even at present the cellulose-dissolving methods that can be used on an industrial scale for this purpose are only the cuprammonium method and viscose method which developed as early as the end of the 1890's. In each of the dopes obtained according to these methods, the cellulose does not dissolve as cellulose itself in the dissolved state but it is dissolved in the form of a certain cellulose derivative. Accordingly, regeneration is necessary for converting this cellulose derivative to the so-called regenerated cellulose. Furthermore, both in the preparation process of dopes according to these methods and in the shaping process from these dopes, many problems arise from the view point of the working environment for labors and environmental pollution because heavy metals are discharged and toxic gases are generated. Although these methods are not utilized industrially, many cellulose-dissolving methods using a metal complex have been known. As the metal complex used for dissolving cellulose, Cadoxene (cadmium/ethylene diamine/alkali), Coxene (cobalt/ ethylene diamine/alkali), Zincoxene (zinc/ethylene diamine/alkali), Nioxene (nickel/ethylene diamine/alkali) and EWNN (iron/tartaric acid/alkali) are noted. However, these methods are not superior to the cuprammonium method or viscose method because toxic components such as heavy metals and amines are used, and thus, these methods are disadvantageous from an economical viewpoint.
The viscose method using carbon disulfide is now adopted by the majority of enterprises in the industry of regenerated celluloses. However, in Europe and America, manufacturers have become apprehensive about industrial continuation of the viscose method because of the above-mentioned problems, and thus many enterprises withdrew from the viscose rayon industry in the 1960's and 1970's. In order to overcome the defects of the known cellulose-dissolving methods, a method for obtaining novel regenerated cellulose shaped articles such as fiber and film in a closed system by dissolving cellulose directly in an organic solvent has been sought since the 1970's, mainly in Canada and the U.S.A., and as a result, various methods have been developed and proposed. However, these methods use a complicated multi-component solvent system which is very expensive, and none of these methods have been used yet in practice because of toxicity, explosiveness and difficulties in recovering the solvents. Moreover, these newly developed methods are very similar to the viscose method or cuprammonium method in the technical idea of converting cellulose to a certain derivative and dissolving the cellulose in the form of this derivative. For example, in the method of using dimethylsulfoxide/paraformaldehyde, cellulose is dissolved in the form of methylol cellulose, and in the method of using dimethylformamide/N.sub.2 O.sub.4, cellulose is dissolved in the form of cellulose nitrite. Also, in the method of using SO.sub.2 /amine and the method of using dimethylformamide/chloral, cellulose is dissolved in the form of a derivative. As is apparent from the abovementioend history of cellulose-dissolving techniques, it is extremely difficult to dissolve cellulose in a simple and cheap solvent.
It is known that if the degree of polymerization (hereinafter referred to as "DP") of cellulose is extremely low (for example, DP =10), the cellulose is soluble in not only an aqueous alkali solution but also hot dimethylsulfoxide. However, the degree of polymerization of this soluble cellulose is necessary lower than 20, and a shaped article made from this cellulose does not have satisfactory mechanical properties and cannot be put to practical use. The reason why cellulose having an extremely low degree of polymerization is soluble in an aqueous alkali solution is that the polymeric characteristics of cellulose, for example, the molecular configuration defined by the hydrogen bond, are lost. It also is known that an aqueous solution containing about 10% by weight of sodium hydroxide shows a strong swelling action to cellulose having a high degree of polymerization. Journal of Prakt. Chem., N.F., 158, 233 (1941) shows the solubilities of natural cellulose, mercerized cellulose, and reprecipitated (probably, regenerated) cellulose in an aqueous 10% by weight solution of sodium hydroxide. Although the dissolving conditions and polymer concentrations are not described, it is taught that natural cellulose and mercerized cellulose are soluble if the polymerization degree (DP) is up to 400 and reprecipitated cellulose is soluble if DP is up to 1200. However, the descriptions include some optional factors and also includes even a highly swollen gel in the category of a "dissolved polymer". From the results of tracing experiments made by us, it has been found that the solubility of cellulose in an aqueous 10% by weight sodium hydroxide solution at -5.degree. C. to 5.degree. C. is influenced by the polymer concentration and the degree of polymerization. For example, in the case of natural cellulose having a degree of polymerization of 360 (cotton linter), when centrifugal separation (at 20000 rpm for 46 minutes) is applied to the dope in order to remove the gel, it has been proved out that the cellulose is not completely dissolved even at a polymer concentration of 0.5% by weight. Accordingly, it is considered that the term "soluble" used in Journal of Prakt. Chem., N.F., 158, 233 (1941) means that cellulose is dissolved at a very low concentration in the gel-containing state. Such a solution of a low concentration is of no practical value. This can be also confirmed from the fact that an aqueous alkali solution was used for the fractional dissolution of celluloses by their lateral order (see, for example, "Purification and Chemical Reaction of Polymeric Substances", pages 128-132, compiled by the Polymer Association and published by Kyoritsu Shuppan in 1958). This fractional dissolution is an operation of separating cellulose into an alkali-soluble portion and an alkali-insoluble portion according to the molecular weight of the cellulose and the aggregation state of the molecular chains, and the former soluble portion even includes a gel. These facts indicate that it is technically very difficult to dissolve substantially all of a cellulose having a high degree of polymerization at a high cellulose concentration in an aqueous alkali solution having a single alkali composition. In fact, a cellulose/alkali solution has not been utilized as a shaping dope in the history of the cellulose industry.